Model iteration setup

I initially had more than this, but pared it down after a conversation about the first iteration of this feature.

My other work is here: https://github.com/dimagi/Vellum/compare/hq-data-api
https://github.com/dimagi/Vellum/pull/286/files#diff-1fb4fd110b6f6cb13c91da29d0706ba1R102 It's a list